Add trace_service_pause_events flag and related tracing

runtime/bin/vmservice/server.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of vmservice_io;
 
 class WebSocketClient extends Client {
   static const int PARSE_ERROR_CODE = 4000;
   static const int BINARY_MESSAGE_ERROR_CODE = 4001;
   static const int NOT_MAP_ERROR_CODE = 4002;
@@ skipping 109 matching lines @@
 
   Future startServer() {
     return HttpServer.bind(ip, port).then((s) {
       // Only display message when port is automatically selected.
       var display_message = (ip != '127.0.0.1' || port != 8181);
       // Retrieve port.
       port = s.port;
       _server = s;
       _server.listen(_requestHandler);
       if (display_message) {
-        print('VMService listening on $ip:$port');
+        print('Observatory listening on http://$ip:$port');
       }
       return s;
     });
   }
 }